Electrodes of this kind have been described in the Belgian Patent Specification No. 867,876. The metal shank of such electrodes contains the cooling system and is covered by an externally disposed compound of high temperature stability, generally being a continuous coating, hooks in the metal shank being provided to improve adhesion.
Similar electrodes are also shown by the British Patent Specification No. 1,223,162 wherein the entire metal shank is covered with a protective ceramic coating. According to this solution of the problem, efforts are made to ensure that the ceramic coating is as thin as possible and penetrates into the metal shank itself to provide a substantial degree of insulation of the cooling tubes extending therein. These tubes simultaneously function as cooling water duct and provide the electrical connection to the consumable graphite electrode part.
The European Patent Application 79 302 809.3 describes an electrode in which the mechanical contact of the metal shank, disposed laterally and externally of the shank, is supported so as to be insulated with respect to the internally disposed metallic cooling system. The bottom part of the metallic cooling shank is again provided with a ceramic coating, secured by hooks and extending approximately to the height of the screw nipple connection.
Electrodes for arc furnaces are exposed to severe stresses. This is in part explained by the elevated operating temperatures, for example in the production of electrode steel, for which such electrodes are most frequently employed. Losses due to side oxidation are also caused at the electrode tip only in an ideal case. Finally, there is the risk of travelling or lateral striking of the arc which can also take place above the consumable part in the event of defects and can lead to short circuits. Furthermore, the electrodes are subject to different temperatures associated with feed and return of the coolant and in the threadable interconnection region of the consumable part by comparison with the cooled power supply unit. The region of the screw nipple represents a particularly endangered region.
Additional and substantial mechanical stresses result from the insertion of the electrodes, boiling distortion and also result from scrap pieces sliding into the melt.
Due to the stringent requirements made on electrodes, constant improvement is required. It is, therefore, the object of the invention to provide electrodes of high activity having a reduced current drop and reduced voltage drop in the electrical supply leads, with a reduced tendency to be trouble prone but being also easy to manufacture and to repair. Particularly in cases of undesirable shift of the arc or excessive mechanical stresses, such electrodes must allow the arc furnace process to be continued, even in the event of partial damage, in a manner which is improved compared with that of conventional electrodes.